When the number of rows of the image sensors and their size increases, it is essential to pay particular attention to the organization of the matrix of pixels which allows for this increase in the number of rows or size.
In a conventional configuration for a moderate-sized matrix with moderate resolution, the organization is very simple: the matrix comprises X rows and Y columns and there is a row decoder making it possible to select one row out of X and a column decoder making it possible to select one column out of Y.
However, when the number of rows increases, it may be necessary to change the organization in order to more rapidly read all the pixels of the matrix. One solution consists, for example, in reading the matrix simultaneously from the top and from the bottom.
Also, when the size of the sensor increases, a limitation is imposed by the photolithography techniques which do not make it possible in practice to expose all at once the entire surface of the matrix and rather require a piecemeal exposure according to the so-called “stitching” technique consisting, during a photolithography step, in successively exposing several contiguous parts of the integrated circuit. However, an exposure of the surface by “stitching” may pose problems for the production of the row decoding circuits.
The present invention starts from these two issues to provide a solution thereto, but it should be understood that the solution can be implemented to address either the first issue or the second, or both together. We will start from the “stitching” photolithography issue to explain the invention.
In this type of photolithography, suited to chips measuring several square centimeters, it is possible to use several different masks to successively expose, during a photolithography step, several regions of the chip, each with its own pattern, until the entire surface of the chip followed by the adjacent chips on the same wafer are exposed. However, it is also possible, advantageously, to use a single mask to expose several regions of the chip when these regions are strictly identical. This is well suited to the production of the matrix of pixels. The matrix can be considered as a juxtaposition of several identical blocks each comprising a certain number of rows. A mask is defined for a single block, and this mask can be moved over the surface to be exposed as many times as there are blocks.
However, there is a difficulty: if a block includes not only rows of pixels but also the decoding elements that make it possible to address these rows, the blocks cannot all be identical. In practice, the decoder part for one block is not strictly identical to the decoder part for another block. The differences are very small but they do exist.
It would therefore be necessary in principle to expose the row decoder separately from the matrix, which necessitates an additional fabrication step; the matrix would be exposed in successive blocks, whereas the row decoder would use a different specific mask. This specific mask would also have to be of large size because the row decoder occupies the entire height of the matrix.
It is also possible to artificially make all the blocks of the matrix of pixels identical, by making the construction of each pixel more complex: a pixel generally comprises a row selection transistor controlled by a row conductor associated with a determined row of pixels, this row conductor being linked to the row decoder; if the matrix is organized in strictly identical blocks, it is then possible to provide for each pixel to comprise an additional block selection transistor, linked to another row conductor originating from the row decoder. A row of pixels would be controlled in read mode only if the two row conductors are activated. It is then possible to have strictly identical blocks, including identical row decoder blocks. However, then the decoding is more complex and, above all, each pixel is more complex.